Extensometer



March 14, 1950 R. P; L AT'HROP I 2,500,836

I EXTENSOMETER Filed May 3, 1945 2 Sheets-Sheet 1 INVENTOR I POM/'1 Bid/1190 ATTQRNE Mach M, 1956} R. P. LATHROP EXTENSOMETER 2 Sheets-Sheet 2 Filed May 3, 1945 Patented Mar. 14, 1950 EXTENSOMETER Robert P. Lathrop, Bethesda, Md., assignor to The Baldwin Locomotive Works, a corporation of Pennsylvania Application May 3, 1945, Serial No. 591,789

6 Claims. I

This invention relates generally to extensometers for materials testing and more particularly to averaging type extensometers having upper and lower sets of gage points each set of which constitutes a pair of opposed points engageable with a specimen on opposite sides thereof, it being understood that herein the term "extensometer includes compressometers.

In the particular form of averaging extensometer disclosed herein the upper and lower sets of gage points are mounted in axially spaced gage blocks which operate a multiplying lever system for measuring axial movement between the two heads in accordance with tension or compression specimen strains. This general type of extensometer and multiplying lever system is disclosed in O. S. Peters Patent 2,198,041. Occasionally a specimen may have certain special physical characteristics, or the operating conditions for loading a specimen may be of such a nature, that the specimen twists slightly about its lengthwise axis when loaded thereby producing relative rotation, in planes normal to the specimen axis, between the upper and lower extensometer heads. This rotation may be in either of opposite directions and in usual extensometers of the averaging type, such opposite rotation will not have a uniform effect upon the measuring system.

It is an object of my invention to provide improved means whereby torsion in either direction in a specimen under test will have a uniform action on the measuring system of an averaging type extensometer so as to produce consistently accurate measurements of strain.

A further object is to provide an improved averaging type extensometer that is relatively simple in construction, operation and maintenance, is rugged for a precision instrument of such type and is compact combined with ease of application to a specimen while at the same time providing a high degree of accuracy, sensitivity and uniformity of action regardless of whether the specimen has torsional efiects in either direction.

Other objects and advantages will be more apparent to those skilled in the art from the following description of the accompanying drawings in which:

Fig. 1 is a perspective of one form of my improved extensometer;

Fig. 2 is a side elevation of the foreside of the extensometer shown in Fig. 1 but with certain parts omitted for sake of clarity;

Fig. 3 is a side elevation looking at the back side of Fig. 1 in the direction of arrow A thereof; and

Fig. 4 is a perspective, with certain parts omitted for sake of clarity, showing a modified form of my improved extensometer which, for purposes of illustration, is shown laid on oneside.

In the particular embodiments of the invention disclosed herein for purposes of illustration, I have shown in Fig. 1 an extensometer having upper and lower gage blocks I and 2 provided with usual opposed gage points 3, 4, 5 and 9 adjustable for different size specimens as disclosed in said Peters patent. Relative axial movementbetween the gage blocks I and 2 in response to compression or tension strains in a specimen (not shown) is preferably measured by a multiplying lever system consisting of a supporting element 8 whose upper end is pivotally connected to block I by a fulcrum pin 9 while its lower end engages a knife edge II] formed as an integral part of a bell crank, one arm II of which is a pin having pivotal engagement with block 2 in the same manner as pin 9 and the other arm I2 of which constitutes an electrical contact of a follow-up mechanism generally indicated at I3. Springs I4 hold the pin II in contact with block 2 as explained in said Peters patent, while springs 59 in conjunction with an arm 60 lend rigidity to the support of element 8 but permit pin 9 to pivot on block I as necessary about its axis and in the vertical plane. To determine the initial gage length of the specimen, i. e., the axial distance between the gage blocks I and 2, a normally inactive spacer plate I5 is suitably rigidly secured by a pair of pins or screws I5, Fig. 3, to one side of block I while the lower end of said plate has a cut-out corner to provide clearance for a gage length set screw 5 and gage screw H. The side plate I5 is of such length that when lower edge of block 2 is moved upwardly until it coincides with the lower edge of side plate I5, then the axial distance between the upper and lower gage points on the side of the inactive plate I5 is equal to the desired gage length. 1

The screw I! is normally released but may be momentarily fastened, if desired, to hold the heads in their predetermined axially spaced relation until after the extensometer is properly connected to a specimen whereupon the screw I! may be released to allow free axial movement between the gage blocks. Upon release of screw I'I side plate I5 ceases to control the position or motion of blocks I and 2.

Located on the opposite side of. the gage blocks is my improved torsion compensating means .pointse and fi. bloc-k2 upwardly in' thedirectionof arrow 36 so =-that theLtwo-blocks are maintainedin substantial parallelism to; each other. :In the event that the torsion-is in a direction opposite'to arrows BG-ahd -34,-the compensating' action of plates-20 and?! which includes a pair of active plates 20 and 2|. The plate 20 is rigidly secured at its lower end to block 2 as by pins and screws 22 while the upper end has a slidable connection with block l.

through a pin 23 and slot 24 formed in the plate. The plate 2| is rigidly secured by pins and screws 25 to the upper block I while its other end is slidablyfb'ofinected to block 2 by pin 26 and slot 21. Plate "-21 -"is of such length that if its lower edge be set flush with lower edge of block 2, where it may be locked in place by the releasable screw 29, then the axial distance between the upper and lower gage points on the side of the active plates 28 and 2! is equal to the desired-gage lengthahd both sides of the gage block's aie-edually spaced from each other in setting the extensometer for its initial gage length. Howeverin order that that the dummy plate'ISis fully released at lock screw l1 and that-lock screw 29 issimilarly released, -'If torsion now occurs in the specimen so that heads 1 and 2 rotate relative to each other inthe direction of, ;say, arrows -33 and =34 respectively, Fig. '1,-then-head I will rotate up-.

wardly' about gage points 3and' 4inth'e direction of arrow 3-5 by reasonofpinZ-G, Fig. 2, 'of lower block'2 rotating in the direction of arrow'34, 'and therefore, moving the lower endbfplate 21 to the right to produce rotation-in 'a counterclockwise direction about upper gag'epoints 3 and l of upper block I, such movement being transmitted "to the-block through the fixed .pin connection--25. -Similarly,the foregoing relative rotation causesblock l to move inthe -direction ol arrow 33 whereby "pin: 23 moves plate -21] to the I left thereby producing rotation in- 'a counterclockwise direction of the lower b1ock==2 about the gage Thisresults in moving'the lower will =be in=a'clockwise direction to swing-blocks I and-2 downwardly together instead of upwardly asprev'iouslydescribed. For instance, as block l-rotates in the-direction Marrow-3T the pin 23 'would==swingiplate 2U ina "clockwise direction about the lower g'a'ge points whereupon the rigid connection between thisplateand-lower block 2 would cause the lattertoswingdownwardly-about tat-ion about the upper :gage point '4 "with bonsequent rotation of block l in a 'dowrl'ward direction about such gagefipoint. .The'two bl'ocl'asare thus again maintained in substantial parallelism. It will be understood that "it is desired to main- .tain=a-constant distancebetween the seats for pins :land-11excej't for specimenstrain infa'n axial 'direction. However, specimen torsion, if "it occurs during testing, will-relatively rotate the blocks 2 and=3 in horizontal plane's, i. e.-, l-aterally displace *the blocks relative to each other.

"This.

nt on the gage screw bushing M. 'ue'reu body ortion or bushings M and '41 are of 'rror that hasheretofore existed is that the side plate and itsc'onnections heretofore used acted to cause blocks 2 and 3 to be tilted toward or -away from each other upon occurrence of such lateral movement.

This tilting action caused the measuring pin seats to be moved relatively toward "or 'aWa'y frorn each other in addition to the moveri'ier it caused direetly by the lateral displacement.

This additional movement is almost entirely eliminated in my arrangement which maintains the blocks in substantial parallelism when they tend-to tilt in response to torsion. The blocks are made to tilt in the same direction and through equal :a'ngles but-because there will be relative rotation between the-blocks the axes of the upper and lower sets 'of gage points will be relatively angularly displaced and the tilting action will be about these axes. Accordinglmthe blocks cannot tilt in absolute parallelism to each other but this-deviation from parallelism is negligible in its efiect.

From the foregoing mode of operation it is seen that the upper and lower gage-blocks are always -ma intained in substantial parallelism to each other regardless of the direction of relative-rotation between the upper and-lower heads produced by opposite torsional effects in aspecimen. it is, therefore, possible to maintain with -a-hig h' degree of precision a'substantially constant distance between the fulcrumpoints of pins 1 and l l :during torsion of the specimen and hence such torsional'effects will not have any apprecia-ble effect upon the strain measuring leverage system B and 12. This measuring system therefore, left free to respond only to actual changes in strain, disregarding the negligible e'fiect or lateral displacement, between the seats for'measuring pins Band II, upon occurrence of torsion.

"Fz'g. 4 modification-4n this arrangement the pair of active or compensating plates are shown at mandhl inove'rlapping relation to each other in distinction to the side by side relation of plates 20 and'Zl infFigfi The plate lilis rigidly secured at its left endby pins and screws '42 to igage block 43 and is also fitted snugly over a "hollow gage screw bushing 34 within which a gage screw 45 is threaded. The opposite end of plate lnha's a slot-which has alapped fit on a gage screw bushing l'l within Which a gage screw 48 is threaded. A second slot'49 formed in plate gage blo'c'k "5'2 "by dowel pins '53 and a spacer which disposed "just'below the shortened edge 55 of plate 40. A nut on threaded bushi'ngwlfl 'mayjhrmlyrigid 'y-nq u side were 5| against "the shoulder er "bushing"?! thereby to cause plat-e151 to' be rigidly secured-to gage block PinsiiB and the nut and screw bushing 48 also assist-in hbldihg theside plate M. The other end of plate 5! is provided with a slot Behaving-elapsed The "shoulsuch length as to-provideanytlesirable "clearance between the side plates thereby pei mitting huts to be threaded on these bushings and firmly tightened against the shoulder 56 and also against the face of plate 4! as previously described.

The strain measuring levers are omitted from Fig. 4 as are also the adjustable gage point screws except &5 and 48 (shown in section). Such parts and others would, however, be identical to those of Fig. 1 but need not be repeated as the principal feature of Fig. 4 is the overlapping of the active plates.

The operation of Fig. 4 modification is the same as Fig. 1 in that any rotation of gage block 43 in in a plane normal to the specimen axis in the direction of arrow 51 will produce a. resulting rotation of plate 5| about the axis of gage point bushing 48 thereby tipping gage block 52 to the left. Similarly, the foregoing relative rotation 5! in the opposite direction of gage block 52 will produce a resulting rotation on plate at in a coun terclockwise direction about the axis of gage point bushing 44 thereby tipping gage block 43 to the left and thus maintain the two gage blocks in substantial parallelism. Torsional movement of the specimen in the opposite direction will cause each plate to move in a clockwise direction and accordingly tend to tip the gage blocks to the right to maintain them in substantial parallelism. The length of side plates 40 and 5| may be such as to determine the initial setting of the gage length merely by setting the ends of the side plates flush with the outside edges of the two gage blocks in the manner described for Fig. 1.

From the foregoing disclosure of the two modifications, it is seen that I have provided an extensometer that is able to compensate for torsional effects in a specimen for either direction of torsion thereby producing an instrument with a high degree of precision with no sacrifice of its sensitivity or accuracy while at the same time providing ruggedness, compactness and ease of application to the specimen.

It will of course be understood that various changes in details of construction and arrangement of parts may be made by those skilled in the art without departing from the spirit of the invention as set forth in the appended claims.

I claim:

1. An averaging extensometer comprising, in combination, upper and lower gage blocks each having opposed gage points, strain measuring means having points of connection to said blocks, means responsive to relative rotation between said blocks about the axis of a specimen arising from torsional elfects in the specimen to which the extensometer is connected for rotating one of said blocks in a given direction about the gage points of said block, and means for similarly rotating the other of said blocks in the same direction in response to said torsional effects, both of said means for rotating said blocks bein 6 disposed on the same side thereof thereby maintaining the distance between said points of connection of the measuring means substantially independent of such torsion.

2. An averaging extensometer comprising, in combination, upper and lower gage blocks each having opposed gage points, strain measuring means connected to said blocks, and torsional compensating means comprising a plurality of lever arms one of which is rigidly connected at one end to the lower gage block and slidably connected to the upper gage block and the other of which is rigidly connected to the upper gage block and slidably connected to the lower gage block, both of said lever arms being disposed on the same side of said blocks whereby relative rotation between said blocks arising from specimen torsion will cause said lever arms to tilt the gage blocks in the same direction and maintain the same substantially parallel to each other substantially independent of specimen torsion.

3. The combination set forth in claim 2 further characterized in that said lever arms are side plates both disposed on one side of said gage blocks.

4. The combination set forth in claim 2 further characterized in that said lever arms are side plates disposed on the same side of the gage blocks in edgewise relation to each other.

5. The combination set forth in claim 2 further characterized in that said lever arms are plates disposed in overlapping relation to each other on the same side of the gage blocks.

6. An extensometer comprising, in combination, upper and lower gage blocks each having gage points, strain measuring means having overlapping levers the first of which is pivotally connected to the upper block and another of which is pivotally connected to the lower block and a pivotal connection between said levers adjacent to the lower block, yieldable means for maintaining the pivotal connection between said levers and with said lower block, and means for maintaining the pivotal connection between the first lever and the upper block including a cross arm connected to said first lever adjacent the upper block and yieldable means connecting said cross arm to the upper block.

ROBERT P. LATHROP.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,017,876 Tripolitis Oct. 22, 1935 2,198,041 Peters Apr. 23, 1940 2,301,872 Henderson Nov. 10, 1942 

